Transthyretin (TTR) is a 55 kDa homotetrameric protein present in serum and cerebral spinal fluid. The function of TTR is to transport L-thyroxine (T4) and holo-retinol binding protein (RBP). TTR is one of greater than 20 nonhomologous amyloidogenic proteins that can be transformed into fibrils and other aggregates leading to disease pathology in humans. These diseases do not appear to be caused by loss of function due to protein aggregation. Instead, aggregation appears to cause neuronal/cellular dysfunction by a mechanism that is not yet clear.
Under denaturing conditions, rate limiting wild type TTR tetramer dissociation and rapid monomer misfolding enables misassembly into amyloid, putatively causing senile systemic amyloidosis (SSA). Dissociation and misfolding of one of more than eighty TTR variants results in familial amyloid polyneuropathy (FAP) and familial amyloid cardiomyopathy (FAC).
The TTR tetramer has two C2 symmetric T4-binding sites. Negatively cooperative binding of T4 is known to stabilize the TTR tetramer and inhibit amyloid fibril formation. Unfortunately, less than 1% of TTR has T4 bound to it in the human serum, because thyroid-binding globulin (TBG) has an order of magnitude higher affinity for T4 in comparison to TTR. Furthermore, the serum concentration of T4 is relatively low (0.1 μM) compared to that of TTR (3.6–7.2 μM).